The present invention relates to a method for the preparation of diorganopolysiloxane. More specifically, the present invention relates to a method for the preparation of diorganopolysiloxane that yields a very pure diorganopolysiloxane that bears a functional group at only one end of the molecular chain, hereinafter referred to as monoterminal-functional diorganopolysiloxane.
In an application that utilizes the functional group reactivity of monoterminal-functional diorganopolysiloxane, monoterminal-functional diorganopolysiloxane is used as a starting material for graft organic polymer having diorganopolysiloxane chain branches. Diorganopolysiloxane of this type has the general formula EQU R(R.sub.2 SiO).sub.p B
in which R is a monovalent hydrocarbon group, p is an integer with a value of at least 1, and B is the hydrogen atom or a group with the general formula EQU --SiR.sub.2 R.sup.1
in which R is a monovalent hydrocarbon group and R.sup.1 is the hydrogen atom or an organofunctional group. Japanese Patent Application Laid Open [Kokai or Unexamined] Numbers Sho 59-78236 [78,236/1984], Hei 1-131247 [131,247/1989], and Hei 2-92933 [92,933/1990] teach the preparation of this type of diorganopolysiloxane by first subjecting cyclic trisiloxane with the general formula EQU (R.sub.2 SiO).sub.3
in which R is a monovalent hydrocarbon group to nonequilibration polymerization, optionally in the presence of silane or siloxane with the general formula EQU R(R.sub.2 SiO).sub.m H
in which R is a monovalent hydrocarbon group and m is an integer with a value of at least 1, under the effect of a lithium polymerization catalyst with the general formula EQU R(R.sub.2 SiO).sub.n Li
in which R is a monovalent hydrocarbon group and n is an integer with a value of at least zero, and by subsequently terminating this nonequilibration polymerization with acid or halosilane with the general formula EQU R.sup.1 R.sub.2 SiX
in which R is a monovalent hydrocarbon group, R.sup.1 is the hydrogen atom or an organofunctional group, and X is a halogen atom.
However, several drawbacks are associated with the preparation of graft organic polymer using diorganopolysiloxane synthesized by the preparative method described above. Thus, the viscosity of the graft organic polymer product undergoes a sharp increase as the molecular weight of the diorganopolysiloxane increases. Moreover, the graft organic polymer product gels when diorganopolysiloxane is used that has a number-average molecular weight above 10,000.
The present inventor undertook various analyses of monoterminal-functional diorganopolysiloxane and its cyclic trisiloxane precursor. As a result of analysis of the starting cyclic trisiloxane using a Fourier-transform infrared spectrophotometer, the inventor confirmed that silanol-containing impurity was present in this cyclic trisiloxane. Gel permeation chromatographic analysis of the monoterminal-functional diorganopolysiloxane demonstrated a secondary peak on the high molecular weight side of the main peak in addition to the main peak due to the diorganopolysiloxane. It was also found that the ratio of this secondary peak increased with increasing number-average molecular weight for the diorganopolysiloxane.
These results suggested that diorganopolysiloxane having functional groups at both molecular chain terminals, hereinafter referred to as diterminal-functional diorganopolysiloxane, is produced as by-product during the synthesis of monoterminal-functional diorganopolysiloxane by the preparative method outlined above.
This diterminal-functional diorganopolysiloxane by-product is extremely difficult to separate by standard methods from the monoterminal-functional diorganopolysiloxane synthesized by the preparative method described above, and this separation becomes even more difficult at higher number-average molecular weights for the diorganopolysiloxane.
In addition, the preceding analyses also suggested that the presence of the silanol-containing impurity such as a silane, or siloxane that contains at least 2 silanol groups in the starting cyclic trisiloxane is the cause of the secondary production of the diterminal-functional diorganopolysiloxane.
However, the silanol-containing impurity is also very difficult to separate from the cyclic trisiloxane by standard methods, such as distillation, column separation using an adsorbent such as silica, alumina, activated clay, and a molecular sieve. The content of silanol-containing impurity in the cyclic trisiloxane is in fact increased in the particular case of column separation of the cyclic trisiloxane because the cyclic trisiloxane ring is in general quite easily opened by acid.
The inventor carried out extensive investigations in order to solve the problems described above and as a result discovered that a very pure monoterminal-functional diorganopolysiloxane can be produced from the nonequilibration polymerization of cyclic trisiloxane in the presence of a lithium compound by preventing the silanol-containing impurity in the cyclic trisiloxane from participating in the nonequilibration polymerization by preliminarily silylating the silanol-containing impurity in the cyclic trisiloxane with silylating agent. The present invention was achieved based on this discovery.